1. Field of the Invention
The present invention relates to a method for fabricating an EUVL mask.
2. Description of the Related Art
For highly densified semiconductor devices, it is inevitable to prepare an exposure light source with a short wavelength. Therefore, an extreme ultra violet lithography (EUVL) using an extreme ultraviolet beam of a wavelength of 13.5 nm has been developed. Currently, not any lens exists, through which the EUV beam can transmit. Accordingly, the EUVL mask reflects the EUV beam for resist exposure. Specifically, EUVL mask 50 shown in FIG. 7B is a reflection-type mask while conventional photomasks are transmission-type masks. The glass substrate of such photomasks corresponds to reflection layer 52 of the EUVL mask 50, while the shield layer of the photomasks corresponds to absorption layer 56 and oxidized layer 58 of the EUVL mask 50. In the EUVL mask 50, a Mo/Si multilayer film is used as the reflection layer 52 while a Ta compound such as TaBN is used as the absorption layer 56. Between the reflection layer 52 and the absorption layer 56, Si capping layer 54a and buffer layer 54b made of CrN are formed.
FIGS. 7A and 7B are explanatory views of a conventional black defect repairing technique, where FIG. 7A is a partial plane view and FIG. 7B is a side cross-sectional view of FIG. 7A along the line B-B. As shown in FIG. 7A, the absorption layer 56 and the oxidized layer 58 are forced out from the designated pattern in the EUVL mask 50, so that black defect 60 may sometimes be formed eventually. Using such EUVL mask for lithography, the EUV beam cannot be reflected on the black defect 60. Hence, the pattern on an exposed wafer is shifted from the designed value, leading to the deterioration or poor performance of the device profile. When the shift is large, electrical wiring may fall into a short circuit or may come down. So as to avoid such events, JP-A-2005-260057 (patent reference 1) discloses a technique for repairing the black defect on an EUVL mask. The technique for repairing such black defect includes allowing an ion beam to selectively irradiate the black defect under feed of an etching gas such as xenon fluoride from a gas gun. By this technique, over-etching into the Mo/Si multilayer film can be progressed at a minimum, owing to the effect of the halogen-series assist gas on the rate enhancement.
In a course of eliminating the black defect 60 with a halogenated xenon, however, the absorption layer 56 is etched isotropically as shown in FIGS. 7A and 7B. Due to the isotropic etching, the side face of the absorption layer 56 is side etched, disadvantageously leading to a problem of the reduction of the pattern precision of the absorption layer 56. On the side-etched part 57 of the absorption layer 56, the absorption profile of the EUV beam is deteriorated. When lithography is done using the EUVL mask, the pattern on an exposed wafer is shifted from the designed value, leading to the occurrence of the deterioration or poor performance of the device profile.
In view of such problems, the invention has been achieved. It is an object of the invention to provide a method for fabricating an EUVL mask, which is capable of fabricating the pattern of the absorption layer of the EUVL mask at a high precision.